The present invention relates to an austenitic stainless steel, more specifically, an austenitic stainless steel with minimized deformation by heating and cooling treatment after cold working. The steel is suitable for structural members of automobiles.
Austenitic stainless steels have been used for various structures because of their excellent workability, strength, corrosion resistance, and the like. In most cases, they are cold worked prior to use.
In the austenitic stainless steels, work-induced martensite may generate during cold working depending on their chemical compositions. In order to prevent this, the following invention is disclosed.
Publication of Japanese Unexamined Patent Application Hei-8-283915 discloses an invention relating to an austenitic stainless steel, which has improved workability due to adjusting the chemical composition, which reduces the generation of work-induced martensite, and also due to controlling the crystal grain size, which reduces work hardening. However, in this invention, the deformation by heating and cooling treatment after cold working is not taken into consideration at all.
It is reported that austenitic stainless steels deform when annealed at a relatively low temperature after cold working. Such a deformation is explained with several different indicators such as stacking fault energy and martensitic transformation quantity.
For example, the shrinkages during low-temperature heat treatment of cold rolled austenitic stainless steels of SUS 301 to SUS 310S are reported in the following literatures 1 to 4. However, in these non-patent literatures, the quantity of shrinkage is explained only with the stacking fault energy of the steel. The deformation and weldability, which is necessary for structure, of high-Si austenitic stainless steels containing Cu, Mo and the like has not been examined at all. Improvement of such high-Si austenitic stainless steels is an objective of the present invention.    Literature 1: CAMP-ISIJ, vol. 15 (2002)-559    Literature 2: TETSU TO HAGANE, Vol. 81 (1995), No.5, pp. 65–70    Literature 3: TETSU TO HAGANE, Vol. 81 (1995), No.9, pp. 32–37    Literature 4: TETSU TO HAGANE, Vol. 82 (1996), No.10, pp. 37–42
Publication of Japanese Unexamined Patent Application 2001-323341 discloses a stainless steel plate having high strength and improved flatness, in which shape correction is performed by use of the work-induced martensite during cold working and by use of shrinkage due to the reverse transformation from martensitic phase to austenitic phase in low-temperature annealing. However, this literature describes neither the inhibition of deformation by heating and cooling treatment after cold working nor the weldability necessary for structure.